The present invention relates to a projection aligner which is used for exposing a substrate to light through a mask to form a predetermined pattern thereon to manufacture printed circuit boards, liquid crystal display panels and the like in accordance with a photo-lithography method.
Conventionally, as the above-described projection aligner, a scanning type projection aligner has been widely used. The scanning type projection aligner is configured as follows.
That is, an image pattern formed on a mask is projected on a surface arranged substantially parallel with the mask at a substantially equi-magnification using a plurality of projecting optical systems. By moving the mask and the surface with respect to the projecting optical systems, the image of the pattern formed on the mask is formed on the surface. Typically, each projecting optical system projects light within a strip-shaped area. By connecting the plurality of strip-shaped areas, which are exposed to the light from a plurality of projecting optical systems, seamlessly, an image pattern is formed in a relatively wide area.
In order to project an image pattern at the equi-magnification, a predetermined object distance is required. If the lenses of the projecting optical system are arranged along a optical axis which is a single line, the size of the optical system in a direction perpendicular to the mask and the surface becomes relatively large. Accordingly, the apparatus employing such optical systems also becomes large.
Japanese Patent Provisional Publication No. HEI 7-326557 discloses an example of a projecting optical system which is configured to have a relatively small size. According to the publication, an optical axis of an imaging lens is arranged to be parallel with the mask and the surface to be exposed, and a right angle prism is arranged on one side of the imaging lens along the optical axis thereof, and a planar mirror is provided on the other side of the imaging lens. A beam from the mask is reflected by one surface of the right angle prism so that the reflected beam is incident on the imaging lens. The beam passed through the imaging lens is reflected by the planar mirror and is incident on the imaging lens again. The beam passed through the imaging lens twice is then reflected by another surface of the right angle prism and directed to the surface to be exposed. According to such a configuration, the size of the optical system in the direction perpendicular to the mask and the surface to be exposed can be reduced with respect to the same object distance.
The projecting optical system described above should be configured such that the diameter of the lens closest to the right angle prism should be larger than the size of the pattern to be projected if the optical system is an equi-magnification, and telecentric system. In particular, in order to achieve the high imaging performance by compensating for aberrations, it is preferable that the diameter of the lens is twice the pattern size. If a plurality of projecting optical systems are provided to connect the exposed areas seamlessly, the imaging lenses cannot be arranged in a single line, and should have a zigzag arrangement. Accordingly, although a room occupied by the optical systems is smaller in a direction perpendicular to the surface to be exposed, it becomes twice or more of the entire length of the imaging lens in the direction of the optical axes thereof.
Further to the above, there are cases where the projection magnification should be changed in order to compensate for expansion/reduction of the substrate due to the variation of the temperature and/or the magnification error in other processes.
The conventional projection optical system disclosed in the aforementioned publication is configured such that the beam travels through all the lens elements in back and forward directions. Therefore, if some of the lens elements are moved in the direction of the optical axis, the change in the optical characteristics due to the movement of the lens elements exhibit equally in both the object side and the image side. With such a configuration, it is relatively difficult to adjust the projection magnification.